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Lithium Abundances of Nearby Young Solar-Type Stars Based on Optical High-Resolution Spectroscopy

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Lithium Abundances of Nearby Young Solar-Type Stars Based on Optical High-Resolution Spectroscopy A&A 537, A91 (2012) Astronomy DOI: 10.1051/0004-6361/201117133 & c ESO 2012 Astrophysics Lithium abundances of nearby young solar-type stars based on optical high-resolution spectroscopy L.-F. Xing1 and Q.-F. Xing2 1 Department of Technology and Physics, Zhengzhou University of Light Industry, 5 Dongfeng Road, 45002 Zhengzhou, PR China e-mail: [email protected] 2 National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China Received 25 April 2011 / Accepted 4 November 2011 ABSTRACT Context. Lithium has long been recognized as a powerful tool for investigating the internal mixing of low-mass stars. In addition, accurate measurements of lithium abundances in young solar-type stars provide independent and reliable age diagnostics. Aims. To study the relationship between lithium and activity in normal solar type-type stars and determine the effectiveness of lithium and kinematics as activity indicators, we perform a lithium survey of nearby late-type stars. We search for nearby young solar-type stars and investigate the relationship between lithium abundances and X-ray activity by measuring log Lx. Methods. On the basis of high-resolution spectroscopic observations, we derived the lithium abundances of 33 young solar-type stars by comparing the measured Li I λ 670.8 nm equivalent widths with the curve of growth calculations in non-LTE (NLTE) conditions. Results. We obtain the lithium abundances of a sample of young solar-type stars. Using the lithium abundance and X-ray luminosity, Lx, we investigate the relationship between Li abundance and X-ray activity. We find a clear correlation between lithium abundance (log N(Li)) and X-ray luminosity, log Lx, for our sample stars. Conclusions. As the X-ray luminosity, Lx, becomes stronger, the lithium abundance decreases in our sample (11 pre-main-sequence, 15 zero-age-main-sequence, and 7 young main-sequence) stars. The results imply that there is a large number of solar-type stars. A correlation appears to exist between Li abundance and age, thus confirming the presence of very active young stars close to the Sun. Key words. stars: evolution – stars: abundances – stars: solar-type 1. Introduction the amount of Li depletion in a star (Barrado y Navascués & Stauffer 1996;Jonesetal.1997). In addition, there is clear Solar-type stars are low-mass, late spectral-type stars. They are evidence of a correlation between rotation rates and Li abun- similar to the Sun in mass and evolutionary state. Physically, dances in the Pleiades and Hyades, with the fastest rotators hav- they have a broadly similar structure and a convective envelope; ing the highest Li abundance (Tschäpe & Rüdiger 2001; Rebolo however, they do not have completely dominant convection, as & Beckman 1988). In particular, the X-ray surface flux is a found in M dwarfs. Young solar-type stars (a weak-line T Tauri good measure of stellar activity because it shows a clear de- star (WTTS), and low-mass zero-age-main-sequence (ZAMS), pendence on stellar rotational velocity (Preibisch 1997). The en- and young main-sequence (MS) stars) display various forms of hanced X-ray emission displayed by rapid rotators (T Tauri stars activity caused by dynamic processes. and RS CVn systems) relative to main-sequence stars can be Lithium has long been recognized as a powerful tool for in- accounted for by their higher rotational velocities (Bouvier & vestigating the internal mixing of low-mass stars. Since lithium Bertout 1985). Therefore, a correlation can be expected between isotopes in low-mass stars are destroyed by proton capture at low lithium abundances and star activity. temperatures, they allow us to directly probe the depths of outer- To study the relationship between lithium and X-ray activity mixed envelopes. Observations of field stars and open clusters in normal solar-type stars, as well as determine the effectiveness clearly show that the destruction rate of Li depends on the mass, of lithium and kinematics as activity indicators, we performed a age, and chemical composition of a star (Duncan 1981; Spite & survey of lithium over a sample of nearby dwarfs. The aim of the Spite 1982; Duncan & Jones 1983;Cayreletal.1984; Boesgaard current paper is to investigate the relation of lithium abundances & Tripicco 1986; Hobbs & Pilachowski 1986; Rebolo et al. to X-ray activity in terms of log LX. The observations and data 1988). In particular, different authors have proposed empirical reduction are presented in Sect. 2. The analysis of lithium abun- expressions for Li abundance as a function of star age (Rebolo dance is provided in Sect. 3, and the results and discussion are 1989; Boesgaard 1991). presented in Sect. 4. Increasing evidence has indicated that dispersion in Li abun- dances exists among stars with similar ages, metallicities, and 2. Observations and data reduction masses (Soderblom et al. 1993a;Boesgaardetal.1998). This dispersion suggests that some other stellar property is important Spectroscopic observations were carried out for thirteen nights in determining the amount of Li depletion in stars. Observational from January 15 to 17, 2008, October 6 to 8, 2009, January 26 to data clearly indicate that rotation plays a key role in determining 28, 2010, and February 18 to 21, 2011 with the Coude Echelle Article published by EDP Sciences A91, page 1 of 6 A&A 537, A91 (2012) HD 141004 Li I CaI HD 126868 Li I CaI HD 81025 Li I CaI HD 82210 Li I CaI Fig. 1. Examples of the spectra of stars with a range of Li I λ 6707 mÅ Fig. 2. Comparison of our measurement of the Li equivalent width ob- lines. tained with a Gaussian fit and similar measurements obtained by other authors for the same stars (Christian & Mathioudakis 2002;Wichmann et al. 2000; Gregorio-Hetem & Hetem 2002; de Laverny et al. 2003; Spectrograph and a 1024×1024 Tek CCD attached to the 2.16 m Takeda & Kawanomoto 2005;Jasniewiczetal.1999; Montes et al. telescope at the Xinglong station of the National Astronomical 2001). Observatories, Chinese Academy of Science (NAOC). The red arm of the spectrograph with a 31.6 groove/mm grating was used in combination with a prism as the cross-disperser, which pro- three strong lines included in our spectral range (Ni I 664.36 nm, vided good separation between the echelle orders (Zhao & Li Fe I 666.34 nm, and V I 662.48 nm) for all target stars. We then 2001). With a 0.5 mm slit (1.1 arcsec), the resolving power was compared them with those measured in the spectra of stars of of approximately 37 000. The exposure time was chosen to en- similar temperature in IC 2391 and IC 2602, which were old sure that the signal-to-noise ratio (S/N) was higher than 100, and enough (30–50 Myr, Randich et al. 2001) to ensure that their ff the spectral coverage is 580–880 nm. This relatively high reso- spectra are not a ected by veiling. Therefore, we did not attempt lution was judged to be important in view of the relatively weak any veiling correction in our young solar-type stars. lithium lines in some stars. In Fig. 2, we compare our measurement of the Li EWs with First, all observational data were reduced using the Image those obtained by other authors for the same stars (Christian & Reduction and Analysis Facility (IRAF)1software package in Mathioudakis 2002;Wichmannetal.2000; Gregorio-Hetem & the standard fashion, including image trimming, bias subtrac- Hetem 2002; de Laverny et al. 2003; Takeda & Kawanomoto tion, flat-field division, spectrum extraction, and cosmic ray re- 2005;Jasniewiczetal.1999; Montes et al. 2001). The agreement moval. The wavelength calibration was then obtained by taking between the values was good; however, for small EWs, we noted ff the spectra of a Th-Ar lamp. Finally, all spectra were normal- some di erences between the samples of other authors and our ized using a cubic spline fit to the observed continuum. Figure 1 results. We found an agreement of closer than 15% for 8 stars shows the spectra of four sample stars in the range of the Li I λ when compared with the literature data, and we estimated an 670.8 nm line. average error bar of 5% in the EW values at 1σ, resulting mainly To determine the Li I λ 670.8 nm equivalent widths from uncertainties in the continuum placement. (EW(Li)), all spectral lines of the relevant order were averaged ff to find regions in the continuum una ected by metal lines. These 3. Lithium abundances of young solar-type stars regions were then used to fit the continuum with a straight line. The EW (Li) was then determined by fitting a Gaussian curve. 3.1. Effective temperatures and surface gravity The nominal resolving power λ/λ was 105,whichwassufficient The effective temperature of our 33 sample stars was determined to resolve the Li I feature from a nearby Fe I line at 6707.44 Å in from the line-depth ratios of VI λ 6251.83 and Fe I λ 6252.527, all regions. Since we observed bright stars and the seeing con- using the calibrations of Gray and Johanson (1991). The line S/N ditions were generally good, the of our observations was depths were measured with a ruler on a plot of the profiles (Xing high, between 100 and 300. 2010). We also determined the effective temperature for 33 pro- Following Palla et al. (2005), the relationship between − = gram stars from the (B V) color index, using the calibrations of the true and measured equivalent widths (EWs) is EWtrue Casagrande et al. (2006). The effective temperature of program − r r EWmeas(1 ), where is the ratio of the excess to the pho- stars derived from different methods is listed in Table 1.
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